Study of mangrove crabs in selected mangrove forests: The relationship between habitat characteristics and their occurrence.

Abstract

Sri Lanka, despite its small size, possesses a high level of biological diversity. Both the coastal and seashore vegetation covers a good part of the coastline. Mangroves occur near river mouths and on the shores of lagoons. Salt marsh vegetation is found on mud flats, which are periodically inundated with seawater. According to the inventory prepared by FAO/UNDP in 1983, the total area under mangrove vegetation in Sri Lanka was about 8000 ha but in 1992 the forest map estimated it as 6877 ha (1UCN, 1993). About 60% of these mangroves are found in Puttalam, Kalpitiya, Portugal, Dutch Bays and the rest in Trincomalee, Jaffna, Koggala, Kalametiya and Kokillai. High diversity of faunal and floral composition and their interactions can be seen in mangrove forests. These faunal and floral components play a vital role in determining the status of the mangroves. When we consider the importance of mangrove crabs, as a constituent group of the fauna, it seems to have a high contribution towards the health of the mangrove vegetation. The role that mangrove crabs play in determining the structure and function of tropical mangrove forests is, however, an issue, which spans both perspectives. Smith el aI. in 1991 working on Sesarmid crabs in Australian mangrove forests have highlighted the important effects of these invertebrates imposed on the carbon cycling in the mangrove communities, and on the structure of the forests. Most species occurring in non-inundated areas (that could be inundated) make burrows to live. Present study was carried out in Galle, Rekawa, Negombo and Pambala. The results reveal that the densities of the burrows vary significantly according to the zone occurrence (Rhizophora, Excoecciria, Open area and Mixed zone) and the location of the study sites (Galle, Rekawa, Negombo and Pambala) at 5% significant level. Numbers of crabs occupying a 1 m2 space also varied from zone-to-zone and site vise. This variation was also significant at 5% level. Numbers of crabs occupying a 1 m2 space also varied with the time. The physical and chemical parameters such as temperature, RH, pH, salinity also varied with the time of 33 Zoology Special Abstracts observation and the zone occurrence. Correlations between the numbers of crabs occupying a I m2 space and the various physical and chemical parameters of the sites did not have a significant relation at 5% level. There can be a correlation between interaction effects of these parameters and numbers of crabs occupying a I m2 of space, which has to be further investigated. However the results show that the numbers of crabs occupying a 1 m2 space during the daytime and the air temperature was significantly correlated in Open area located in Galle. The soil texture also varied from zone-to-zone in Galle. But a uniform muddy mixed sandy substrate was observed in Rekawa. Number of crab species found in Rhizophora zone, Excoecaria zone and Open area are respectively 13, 4 and 8 in Galle. Carcinoma carnifex was found in both Excoecaria zone and Open area while Neosamatium meinerti was found in both Rhizophora and Excoecaria zones in Galle mangrove forest. Number of species found in the Mixed zone in Rekawa, Negombo were 3 and 4 respectively, where as that recorded in Excoecaria zone in Pambala was 4. The density of burrows also varied significantly depending on the location. According the mean number and the standard error of the density of the burrows recorded at the Mixed zone in Rekawa and Negombo were 2.1 ± 0.1667 and 7.3333 ± 0.4216 respectively. While that recorded in Excoecaria zone in Pambala was 7 ± 0.4472 m A significant difference between the number of crabs occupying in a 1 m2 space during daytime in Excoecaria zone in Galle and Pambala was also evident (t = 4.727, df = 17, p < 0.001) and there was also a significant difference between the numbers of crabs in 1 m2 of space during daytime in Mixed zones in Rekawa & Negombo (t = -3.352, df = 22, P < 0.01) was evident.